The field of nonlinear optics has continued to develop since the early 1960s and over time has yielded solutions to many challenges encountered by optical devices and systems including generation of light at difficult to reach or inaccessible frequencies. At present, nonlinear optics encompass a wide variety of phenomena mediated by materials that are susceptible to being polarized by optical radiation. These kinds of materials are frequently called nonlinear optical materials and their susceptibility is referred to as a nonlinear optical susceptibility χ. Suitable nonlinear materials include nonlinear crystals having a second-order susceptibility χ(2) or higher order susceptibilities, as well as attractive optical and mechanical characteristics.
Nonlinear optical interactions occur whenever the optical fields associated with one or more beams of optical radiation, typically supplied by a laser, are large enough to produce significant polarization fields in a nonlinear material via the χ(2) or higher order susceptibilities. The polarization fields thus produced are proportional to the product of two or more of the incident optical fields. Currently, such nonlinear optical processes are being used to generate new frequencies in various nonlinear processes mediated by second- and higher-order susceptibilities. These processes include three- and four-wave mixing. The most common three-wave mixing processes mediated by the second-order susceptibility χ(2) are performed in nonlinear crystals and include second harmonic generation, sum frequency generation, difference frequency generation, optical parametric generation, optical parametric amplification, optical parametric oscillation and cascaded combinations of these processes.
The nonlinear polarization fields created in a nonlinear material radiate electric fields at a frequency, e.g., the second harmonic frequency, sum frequency or difference frequency. The strength of these radiated fields grows linearly with interaction distance. Therefore, efficient nonlinear frequency mixing is achieved when the interacting nonlinear polarization fields and the nonlinear frequency of the radiating electric field are maintained in phase over a long distance. For this reason, considerable efforts have been devoted during the last decade to understanding the propagation of electromagnetic waves in nonlinear materials including crystals and phasematching the nonlinear interactions over the longest possible distances. For a review of these efforts the reader is referred to Robert Boyd, Nonlinear Optics 2nd edition, Academic Press (San Diego, 2003), Sections 2.7 and 2.9.
A prominent technique for achieving phasematching in nonlinear optical interactions is quasi-phasematching (QPM). This method is typically implemented in periodic gratings produced by reversing the sign or otherwise modulating the nonlinear optical susceptibility of the nonlinear material in the regions or domains making up the grating. Nonlinear materials in which such gratings can be successfully produced include, among other, periodically poled LiNbO3 (PPLN) and LiTaO3. For more information on the theory of QPM, QPM gratings and their applications the reader is referred to Martin M. Fejer, “Nonlinear Optical Frequency Conversion”, Physics Today 47, 25-32 (May 1994), Martin M. Fejer, G. A. Magel, D. H. Jundt, and R. L. Byer, “Quasi-Phase-Matched Second Harmonic Generation”, IEEE J. Quantum. Electron. QE-28, 2631-2654 (November 1992) or Larry E. Myers, R. C. Eckardt, M. M. Fejer, R. L. Byer, W. R. Bosenberg and J. W. Pierce, “Quasi-Phasematched Optical Parametric Oscillators using Bulk Periodically Poled LiNbO3, Journal of the Optical Society of America B, Vol. 12, pp. 2102-2116 (November 1995).
Standard nonlinear frequency mixers employing QPM gratings to generate output light at a desired frequency still lack many features necessary to make them more universally accepted. In one aspect, frequency mixers with QPM gratings are typically designed to operate within a narrow band of frequencies. In other words, these devices have a narrow frequency tuning range. To overcome this limitation extended tunability or frequency-agile frequency mixers using PPLN have been proposed by implementing QPM gratings of different periods in a side-by-side configuration. An exemplary approach is described by L. E. Myers, R. C. Eckardt, M. M. Fejer, and R. L. Byer, “Multigrating Quasi-Phase-Matched Optical Parametric Oscillator in Periodically Poled LiNbO3”, Optics Letters, Vol. 21, pp. 591-593 (April 1996). In accordance with another approach, a transversely patterned periodic grating for QPM has been used to increase the tuning range by producing a transverse fan-out in the QPM grating. This approach is taught by Y. Ishigame et al., “LiNbO3 Waveguide Second-Harmonic-Generation Device Phase Matched with a Fan-out Domain-inverted Grating”, Optics Letters, Vol. 16, No. 6, pp. 375-377 (1991) and by P. E. Powers et al., “Continuous Tuning of a Continuous-Wave Periodically Poled Lithium Niobate Optical Parametric Oscillator by Use of a Fan-out Grating Design”, Optics Letters, Vol. 23, No. 3, pp. 159-161 (1998).
Another limitation of frequency mixers with QPM gratings is their inability to selectively adjust parameters of interacting light beams and/or of the output light beam(s). To overcome this limitation the publication of G. Imeshev et al., “Lateral Patterning of Nonlinear Frequency Conversion with Transversely Varying Quasi-Phase-Matching Gratings”, Optics Letters, Vol. 23, No. 9, pp. 673-675 (1998) describes a method for shaping the amplitude profile of an interacting beam by introducing a transverse variation in the QPM grating. Furthermore, efficient non-collinear mixing of interacting beams is provided for by two-dimensional periodic QPM structures including hexagonally-shaped domains described by N. G. R. Broderick et al., “Hexagonally Poled Lithium Niobate: A Two-Dimensional Nonlinear Photonic Crystal”, Physics Review Letters, Vol. 84, No. 19, pp. 4345-4348 (2000). Similar goals can be achieved by domains of various shapes (including hexagonal) arranged in lattices, e.g., triangular lattices, as described by V. Berger, “Nonlinear Photonic Crystals”, Physical Review Letters, Vol. 81, No. 19, pp. 4136-4139 (1998).
Although the patterning of QPM gratings in accordance with the above-mentioned prior art techniques has rendered QPM gratings more frequency-agile and versatile in managing the nonlinear mixing of interacting beams, there are still many operations that these devices cannot perform. Specifically, present day frequency mixers with QPM gratings cannot selectively shape and/or modulate the interacting light beams and especially the output light beam(s). Thus, any beam shaping, focusing or modulating functions have to be carried out by additional optical elements positioned outside the frequency mixer.